Photographic imaging systems that utilize differences in tackiness between exposed and unexposed areas of light-sensitive layers are known in the photographic reproduction area. Photohardening and phototackification are two alternate means of achieving the required tacky/nontacky differential between exposed and unexposed areas.
Reproduction systems utilizing photohardening are known from U.S. Pat. Nos. 3,060,024; 3,620,726; and 3,582,327, wherein a tacky photopolymerizable reproduction material is hardened by imagewise exposure so that the exposed image areas lose their tackiness. The image is then made visable by dusting on of suitable toners, which adhere only to the unexposed tacky areas, but do not adhere to and can be removed from the exposed, nontacky image areas. This process yields a positive image of the original.
Reproduction systems using phototackification are also known. For example, a negative tonable material which contains a combination of at least one dihydropyridine compound and at least one bisimidazole compound is known from U.S. Pat. No. 4,243,741. The image production is based on the known photochemical dissociation of bisimidazoles. The low molecular weight cleavage products formed thereby bring about an increase in the tackiness of the exposed areas, so that a toner will then adhere to the exposed areas. Another system based on the same principle is described in U.S. Pat. No. 4,346,162.
However, in order to obtain satisfactory reproduction quality with bisimidazole systems, it is necessary to prevent the recombination of the cleavage products. A recombination would lead to an impairment of sensitometric properties as well as an inability to achieve the desired coverage with toner powder on the exposed areas. Recombination can be prevented, however, by an internal or external plasticization of the layers which leads to a reduction of viscosity so that a minimum mobility of the cleavage products in the layer is assured. It is disadvantageous, however, that such layers have a tendency to form stain by deposition of toner in unexposed areas. Additionally, many plasticizers tend to diffuse or to evaporate from the layers, resulting in poor storage stability, variable sensitometric properties, and primarily varied or even insufficient densities of toner when different exposure and toning steps are carried out on the same light-sensitive layer.
Another negative tonable reproduction material is known from U.S. Pat. No. 4,273,842. The light-sensitive diazonium salts described therein form hygroscopic decomposition products on exposure to light. These decomposition products then become tacky by absorption of water. A uniform covering of the exposed areas with toners with this material is only attainable when exposure and toning are carried out in the presence of constant amounts of water. Therefore, expensive apparatus are required to establish and control the humidity in the room and the length of exposure to the air in the room.
In addition, the diazonium salts contained in the photosensitive layers must be in a crystalline state at least on the surface of the layer. Otherwise, the layers are tacky even before exposure. This leads to poor differentiation between exposed and unexposed areas, as well as fogging and the formation of spots. A further disadvantage which renders tacky starting materials useless for many applications is their sensitivity to dirt and dust before exposure.
An object of the present invention is, therefore, to find a process for the production of patterns comprising powders on a substrate using a negative-working tonable light-sensitive layer, which even upon multiple exposures achieves a high density and uniform coating of the exposed areas with toners. Other objects are to avoid changes in photographic sensitivity brought about by varying concentrations of plasticizer and to allow exposure and toning to be carried out under normal room conditions.